Image intensifier and method of making an electron multiplier therefor

ABSTRACT

The invention includes a channel-type electron multiplier incorporating a glass plate having holes therethrough. The plate is treated so that the holes will support secondary emission. When the primary electrons from a photo-cathode enter the holes, an increased current density is produced at the output of the holes. When the output is directed onto a phosphor screen, an intensified image of the scene being viewed is produced. Night scenes may thus be brightened for use in military reconnaissance or the like. However, channel-type electron multipliers have what is known as a saturation level of operation beyond which an increase in input current produces little or no increase in output current. The device of the present invention alleviates this problem by increasing electrical resistance near the output. This construction makes it possible to produce a special electrical field distribution in the holes. This field distribution then causes the gain of the multiplier to saturate at substantially higher input current levels. In addition, a multiplier of this type will exhibit a nonlinear input versus output relationship such that, if incorporated in an image tube (direct view or TV pickup) it imparts to this tube highly desirable contrast enhancing characteristics. The invention also includes a method of making the special type of electron multiplier having the hole surface resistivity variation.

R. K. ORTHUBER 3,788,831 IMAGE INTENSIFIER AND METHOD OF MAKING .ANELECTRON MULTIPLIER" THEREFOR Jan. 29, 1974 4 Sheets-Sheet 1 Filed Feb.22, 1971 FIGZ n'lllllllil'lllld I I I I I I I n I FIGS) INVENTOR.

RICHARD K. ORTHUBER FIGI ATT RNEY Jan. 29. 1974 R. K; ORTHUBER 3,788,831

IMAGE INTENSI'FIER AND METHOD OF MAKING AN ELECTRON 'MULTIPLIER THEREFORFiled Feb. 22, 1971 j 4 Sheets-Sheet :1

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ATTORNEY K. OIRTHUIBER 8 3,788,831

Jan. 29, 1974 R IMAGE INTENSIFIER AND METHOD OF MAKING AN ELECTRONMULTIPLIER THEREFOR 4 Sheets-Sheet 5 Filed Feb. 22, 1971 x 6 m D G M l MF. M T

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Jan- 29. 1974 R. K. ORTHUBER. 8 1

IMAGE INTENSIFIER AND METHOD OF MAKING AN ELECTRON MULTIPLIER THEREFORFiled Feb. 22, 197-1 4 Sheets-Sheet 4 SOURCE OF FIG.|4

v INVENTOR. RICHARD K. ORTHUBER A TTOR/VE Y US. Cl. 65--111 5 ClaimsABSTRACT OF THE DISCLOSURE The invention includes a channel-typeelectron multiplier incorporating a glass plate having holestherethrough. The plate is treated so that the holes will supportsecondary emission. When primary electrons from a photocathode enter theholes, an increased current density is produced at the output of theholes. When the output is directed onto a phosphor screen, anintensified image of the scene being viewed is produced. Night scenesmay thus be brightened for use in military reconnaissance or the like.However, channel-type electron multipliers have what is known as asaturation level of operation beyond which an increase in input currentproduces little or no increase in output current. The device of thepresent invention alleviates this problem by increasing electricalresistance near the output. This construction makes it possible toproduce a special electric field distribution in the holes. This fielddistribution then causes the gain of the multiplier to saturate atsubstantially higher input current levels. In addition, a multiplier ofthis type will exhibit a nonlinear input versus output relationship suchthat, if incorporated in an image tube (direct view or TV pickup), itimparts to this tube highly desirable contrast enhancingcharacteristics.

The invention also includes a method of making the special type ofelectron multiplier having the hole surface resistivity variation.

BACKGROUND OF THE INVENTION This application is a division of copendingapplication Ser. No. 815,519, filed Apr. 8, 1969, for Image Intensifierand Method of Making an Electron Multiplier Therefor. The benefit of thefiling date of said copending application is, therefore, hereby claimedfor this application.

The invention relates to electron multipliers and, more particularly, toa channel-type electron multiplier having an unusually high saturationlevel.

In the past, image intensifiers have incorporated a transparent,evacuated envelope having a phosphor screen on an output side of achannel-type electron multiplier and a photocathode on the input sidethereof. A scene illuminating the photocathode causes primary electronsto enter holes of channels in the multiplier. The multiplier includes aperforated glass plate which has been heated in a hydrogen atmosphere totreat the channel surfaces in a manner to cause them to supportsecondary emission.

When the primary electrons bombard the channel surfaces, secondaryelectrons are released at a ratio greater than unity. The secondariesare accelerated part way down a channel and bombard it again. Thisprocess is then repeated over and over again so that the current densityat the output of the multiplier is much greater than that at its input.A bright image of the scenes being viewed is then displayed on thephosphor screen. Night scenes may, thus, be displayed with substantialbrightness for military reconnaissance or other purposes.

Conventional channel-type electron multipliers have evaporatedconductive input and output coatings on the United States Patent 03,788,831 Patented Jan. 29, 1974 input and output sides, respectively,of the dielectric plates. These coatings have holes therethrough inregistration with the glass plate holes. The coatings act as electrodes.The output coating or electrode is maintained positive with respect tothe input electrode. These electrodes at these relative potentials,thus, create a field in the channels and produce an electron current atthe output of the multiplier. Electrons also flow from the negativeelectrode in the channel surfaces to balance positive wall chargescaused by the emission of secondaries.

When the input current to a conventional channel-type multiplier isincreased from zero, the output current rises approximately inproportion to the input current, depending upon the gain of themultiplier. However, above a predetermined input current, the gain ofthe multiplier tends to decrease with increasing input. This conditionis called saturation. It is believed that this condition exists becausethe channel surfaces near the output ends thereof become depleted ofelectrons. This, in turn, is believed to cause an electric fielddistortion that reduces the multiplier gain at elevated input and outputcurrents.

The saturation condition of a conventional channel-type multiplier is astrict limitation of the output current density of an image intensifier.

SUMMARY OF THE INVENTION In accordance with the device of the presentinvention, the above-described and other disadvantages of the prior artare overcome by providing a channel-type electron multiplier in whichthe channel Wall resistance near the input end of a channel is less thanthat at the output end thereof. It is believed that the inventionincreases the gain at elevated input and output currents because thespecific wall resistance provided in accordance with the device of thepresent invention corrects the said electric field distortion. At anyrate, the multiplier of the invention has a relatively low gain atmoderate current inputs and a relatively high gain at elevated inputcurrents. This makes it possible to cascade the multiplier of theinvention with a conventional multiplier. The conventional multipliercan then be operated to produce an output current for the input to themultiplier of the invention. The conventional multiplier may, thus, havea high gain at a low input current, and the multiplier of the inventionmay have a high gain at its elevated input current. Saturation, thus,limits the maximum gain of cascaded multipliers in accordance with theinvention to a lesser degree as was the case in the prior art.

The invention also includes a method of making the multiplier of theinvention.

The above-described and other advantages of the invention, particularlythe possibilities of contrast enhance ment, will be better understoodfrom the following description when considered in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, which are to beregarded as merely illustrative:

FIG. 1 is a sectional view of a conventional image intensifier tube;

FIG. 2 is a greatly enlarged sectional view of a channeltype electronmultiplier;

FIG. 3 is a graph of a current characteristic of a conventionalmultiplier;

FIG. 4 is a graph of the potential distribution along the axis of aconventional multiplier;

FIG. 5 is a graph that is an approximation of a potential distributionshown in FIG. 4;

FIG. 6 is a graph of an approximation of the potential distribution of amultplier made in accordance with the present invention;

FIG. 7 is a graph of both current characteristics demonstarting theshift in saturation level with the multiplier of the invention;

FIG. 8 is a graph of a current characteristic indicating how contrastmay be enhanced with the multiplier of the invention;

FIG. 9 is a sectional view of portions of a multiplier to be constructedin accordance with the present invention;

FIGS. 10, 11 and 12 are sectional views of apparatus which may beemployed in the treatment of the glass plate of a multiplier inaccordance with the invention;

FIG. 13 is a sectional view of a multiplier constructed in accordancewith the invention; and

FIG. 14 is a sectional view of an image intensifier constructed inaccordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the drawings in FIG. 1, aconventional image intensifier tube is shown including a transparent,evacuated envelope 11. A photocathode 12 and a phosphor screen 13 arepositioned within the envelope 11 on opposite sides of a channel-typeelectron multiplier 14.

Multiplier 14 is shown greatly enlarged in FIG. 2 including a perforateglass plate 15 having perforate, evaporated, conductive coatings 16 and17 thereon. Plate 15 is conventionally treated by heating in a reducingatmosphere to make the internal surfaces of channels or holes 18 capableof supporting secondary emission.

If photocathode 12 is positioned continguous to input electrode 16 andscreen 13 contiguous to output electrode 17, a brightened image of thescene being viewed will appear on screen 13. Typical operatingpotentials are: Input electrode 16, 1,000 volts; output electrode 17,zero volts; screen 13, +5,000 volts; photocathode 12, -l,100 volts.

Conventional image intensifier tubes using channeltype electronmultiplier plates for intensification show a relationship between outputbrightness and input brightness or output and input current densities jand i which is described by a characteristic of the form shown in FIG.3.

For low values of output density jg, this characteristic shows a linearincrease of the output density with and, thus, constant gain G =tan 00.At higher levels, however, approaches a saturation level of outputcurrent density the characteristic bends into a horizontal direction andj, This saturation phenomenon is caused by the following fact.

As the emission density of secondary electrons increases (particularlynear the output end of the channel walls) and approaches the stripcurrent, i.e., the conduction current along the semiconductive channelwalls, the potential within the channels ceases to increase uniformlytowards the channel walls as it would in the absence of an electroninput. However, close to the output end where the secondary emissiondensity is highest, this potential drifts to a more positive than normalstate due to the inability of the limited strip current to completelyreplace the charges lost by secondary emission.

This phenomenon is discussed, and the above hypothesis for the cause ofsaturation is qualitatively substantiated in Review of ScientificInstruments by D. S. Evans, vol. 36, No. 3, March 1965, pp. 375-382.This article illustrates the potential rise along a channel axis for achannel in the absence of an electron cascade (curve A-C) and for aloaded channel (curve A-B-C) as shown in FIG. 4.

The uniform field represented by the unloaded curve A-C is seen tobecome distorted as the electronic load is applied in such a fashionthat in a large section of the channel adjoining the input aperture, aslight increase in field results. In a short section near the outputaperture, the field is drastically reduced and nearly disappears(horizontal section B-C).

: It can be shown, using a simplified model of FIG. 4, that a transitionfrom curve A-C to curve A-B-C results G=e (a% for the gain of a channelsection of length A (where a and b are constants involving channelradius and secondary emission properties, and E is the field gradient inthe channel), the gain for the two sections in FIG. 5 becomes:

The total gain:

This shows that for fi- 0 or 18-)1, the total gain G tends toward zero,the case 3+1 representing output saturation as illustrated by FIG. 4. Anintermediate value must, therefore, represent maximum which is formedfrom dG/dp=0.

Inspection of FIG. 5 shows immediately that 5:00 implies a gradientuniform throughout the channel. The deviation 3 oo induced by residualwall charges leads to saturation as stated before. The objective of thisinvention is an exploitation of the interdependence of gain and fielduniformity for the purpose of contrast enhancement.

For this purpose, channels are used in which:

( 1) The longitudinal gradient is varying along the axis in the absenceof electronic load;

(2) Varies in the opposite sense as that in a conventional channeloperating in or near saturation.

This means in the unloaded state fi oo.

For such a channel, the potential diagram of FIG. 5 for a uniform fieldchannel is changed to that shown in FIG. 6.

In FIG. 6, the potential distribution in the case of no or low electroninput is qualitatively shown by the solid line fl eo. This distributionmay be established by means of a channel plate in which the conductivityof the channel walls is higher in the input section than in the regionclose to the output end. According to the prior discussion herein, thischannel would have less gain than an otherwise similar one with uniformfield distribution.

We consider now the electronic loading applied to increase until underthe eflfect of non-neutralized wall charges the potential near 00L hasincreased to the value pV =wV i.e. 00: 9. In this state, a higher gainwould be reached than with no or a low electronic load according to thepreceding discussion.

We consider the load increased still further to make p oe. Then, thischannel will saturate as a conventional uniformly-conductive channel.The response characteristic for such a channel of the inventon is,therefore, different from that shown in FIG. 3 for a conventionalchannel and shaped as shown in FIG. 7.

Whereas the conventional channel has a maximum and constant gain for allbut very high levels, the channel of the present invention shows avariable slope of this response characteristic with a toe and shouldersomewhat similar to the H and D chracteristics familiar in photography.The first with strict proportionality between input and output transmitscontrasts unchanged except if driven to saturation. The second, however,has the capability of transforming low contrasts in the input image intohigh display contrasts, "a capability which was so far restricted tophotographic or TV-type electronic processing. The possibility ofcontrast enhancement is illustrated in FIG. 8 which shows a transfercharacteristic of a channel of the present invention with the slope Gsignifying the total gain for an input 11 and 'y, the maximum slope ofthe characteristic, which determines the differential gain, i.e., theratio of an output brightness increment to the corresponding incrementof input brightness.

A multiplier plate formed by an array of such channels of the inventionand operated with an electron image having an input current density j inthe highlights and j; mm in the lowlights and, thus, an input contrastC1 j1 max.' j1 min. jl

jl max. ,71 max.

will then produce an intensified output image with contrast .72 max.

.71 max.

(1) Adjustment of the voltage across the channel plate of the presentinvention;

(2) Adjustment of lens speed;

(3) Gain adjustment of a conventional channel plate arranged ahead ofthe plate of the invention; and

(4) Gain adjustment of a conventional linear intensifier coupled to thecontrast enhancing intensifier.

It remains how to disclose ways to achieve the required non-uniformconductivity along the walls of the plate of the invention, i.e., amosaic formed by many channels of the invention.

The first approach to an acceptable contacting method for a high and lowconductivity plate would require contact over the total areas of thecontacting surfaces of two dielectric plate sections. In order tofacilitate such a contact, the front plate on its output surface isground or sagged to a very slightly spherical shape, whereas the backplate on its input surface is ground and polished flat, as shown in FIG.9. The second plate may be significantly thinner than the first, sincethe break of field strength occurs close to the channel ends.

In FIG. 9, input and output electrodes 19 and 20, respectively, areconventional, vacuum-deposited, contiguous electrodes and connected tothe power supply. On squeezing the two plates together by applyingpressure to their rims, the second plate will wrap over the first plate,establishing a contact pressure more uniformly distributed over the areathan would be the case if both plates were flat.

Omission of metallic electrodes between plates 21 and 22 may give riseto non-uniformity of contact resistance over the interface of thearrangement of FIG. 9. As a solution to this transition-resistanceproblem, low resistivity is provided between the contacting surfaces,while preventing lateral conductivity along the surfaces in a directionnormal to the channel axis.

Metallic electrodes with this characteristic are formed by depositingmutually-insulated, conductive islands indicated at K on the contactingsurfaces, either by evaporation through very fine mesh metal screens orby techniques known from the preparation of iconoscope targets, e.g., bydeposition of a contiguous silver-film which is then broken up intoseparate conductive islands exhibiting high resistance in a transversedirection.

Another way of preparing the multiplier of the invention starts out witha single structure and accomplishes non-uniform conduction bydifferential activation.

Conventional channel-type multiplier plates are usually provided withthe required conductivity and secondary emission properties bysubjecting the normally highly resistive base structure of a reducableglass to a hydrogen firing process. The duration and temperature atwhich the resulting reduction of the channel wall surface is carried outhave a pronounced bearing on the condition of the resultingsemiconductive layer formed on the channel walls. For moderate durationof the process, e.g., less than twenty hours at around 400 degreescentigrade, the resulting conductivity increases with the temperature ofthe glass during activation.

Non-uniform conductivity within multiplier channels can be accomplishedby establishing, during activation, at temperature gradient through achannel plate in the direction of the channels. The following twoprocedures establish this temperature gradient during activation and maybe used singly or in conjunction. The first is illustrated in FIGS. 10and 11.

FIGS. 10 and 11 show a quartz cylinder 23 mounted within an oven whichcontains independently-adjustable heater elements 24 and 25. Within thequartz cylinder 23, a bafliing element 26 is mounted so that it runsparallel to the axis of quartz cylinder 23. The bafiiing elements 26contain one or more apertures onto which one or more non-activatedchannel plates 27 are mounted. Hydrogen admitted from the bottom of FIG.11 has to pass the channels of the channel plates in the baffle in orderto leave through the outlet on top of FIG. 11.

This is all as in conventional channel plate activation. In order toobtain the channel plate of the invention, the outer heaters areenergized to different levels, e.g., the right-hand heater 25 higherthan the left-hand heater 24. In this case, the heat flow onto theright-hand surface of the channel plate exceeds that onto the left-handsurface; and heat will flow between the two channel plate surfaces,establishing a temperature gradient through the plate so that theright-hand surface is hotter than the left-hand surface. Consequently,activation and conductivity buildup will proceed at a more rapid rate atthe right-hand channel ends; and the end result will be channels withwall conductivity increasing from the left to the right, which resultsin a channel plate usable for contrast enhancement.

An alternate procedure is illustrated in FIG. 12. The heaters arearranged as in FIGS. 10 and 11. Differential heating by the heaterelements may or may not be applied. Again, activation occurs in a quartztube 28, which is again separated into two halves by a baffle 29containing the channel plate 30 under activation. Gas exchange betweenthe two halves occurs only through the channel plates 30.

In contrast to FIGS. 10 and 11, differential heating of the plate occurshere by two separate hydrogen flows entering the two chambers of theactivation vessel. The gases entering the chambers are preheated in heatexchangers 31 and 32 so that the gas entering the right-hand V. K.Zworykin and G. A. Morton, Television, 2d ed., Wiley Sons, 1940, 1945,pp. 310-311.

7 chamber has a higher temperature than that entering the left chamber.This results again as in FIGS. 10 and 11 in a temperature gradientthrough the channel plate 30 and thus non-uniform activation of thechannels.

If the pressure in both chambers is kept equal so that no net flowthrough the channels occurs, the hot and cold hydrogen will mix in thechannels by diffusion so that a gradual temperature drop of the gas fromright to left is established. The temperature profile within thechannels can then be shifted by adjusting the pressures in the twohalves at different levels, inducing a net flow in the channels. If theright-hand chamber pressure is increased above the pressure in the leftchamber, a flow of hot gas through the channels will be superposed tothe difiusion process. The highly-activated segments of the channelswill expand more if the pressure excess of the hot gas chamber ishigher. In this way, the method of FIG. 12 permits the establishment ofcontrollable conductivity profile in the channels.

In accordance with the present invention, a channeltype electronmultiplier 33 is provided as indicated in FIG. 13, including a glassplate 34 made of two layers according to FIG. 9 or by the other methodsdisclosed herein. Conventional input and output electrodes 35 and 36,respectively, may be provided. However, channel surfaces 37 of plate 34have a resistance at the input ends thereof less than that at the outputends. Preferably, the length of the surfaces of greater resistance issubstantially less than the length of the surfaces of less resistance tosimulate the unloaded curve of FIG. 6.

According to the present invention, an image intensifier tube 38 isshown in FIG. 14 including an evacuated envelope 39, a photocathode 40,two channel-type electron multipliers 41 and 42, and a phosphor screen43. Multipliers 41 and 42 are, thus, cascaded. Multiplier 42 isconstructed in accordance with'the present invention to have a currentcharacteristic approximately the same as that shown in FIG. 8.Multiplier 41 is conventional and is operated to give an output currentapproximately equal to mm shown in FIG. 8, although the output currentof multiplier 41 may be greater or less than This, then, makes itpossible to operate multiplier 42 in the higher input current-maximumgain portion of its current characteristic between i mm and max, orhigher.

A source of potential 44 maintains the conductive por- 8 tions of tube38 inside envelope 39 at appropriate operating potentials.

It should be mentioned that the channel plate of the invention is notrestricted to use in a direct view tube displaying an electron image ona phosphor. It is also advantageously applicable to tubes involvingother processing steps for the output electron image. These may be, forexample, image storing tubes, TV pickup tubes, or image dissectors.

What is claimed is:

1. The method of making a channel-type electron multiplier, said methodcomprising the steps of: forming a dielectric plate with holestherethrough between opposite sides of said plate; and heating the platein an atmosphere of hydrogen gas in a manner to cause a temperaturegradient to exist along said holes from said one side of said plate tothe other.

2. The invention as defined in claim 1, wherein said plate is heated bytwo different heaters on said opposite sides thereof, said hydrogen gasbeing circulated from the hottest side of said plate to the coolest sidethereof through said holes.

3. The invention as defined in claim 1, wherein said hydrogen gas issupplied on each side of said plate at two different temperatures.

4. The invention as defined in claim 3, wherein said gas is supplied toeach side of said plate at the same pressure.

5. The invention as defined in claim 3, wherein said gas is supplied toeach side of said plate at a different pressure.

References Cited UNITED STATES PATENTS 2,237,242 4/1941 Tykociner et a1....1 316-13 X 2,497,911 1/1950 Reilly et al. 316-13 X 2,424,294 7/1947White 316-24 2,821,637 1/195'8 Roberts et al. 313-68 X 3,497,759 2/ 1970Manley 315-11 BENJAMIN A BORCHELT, Primary Examiner J. M. HANLEY,Assistant Examiner U.S. Cl. X.R.

